Activity diagram

Activity diagrams are the UML diagram for processes. Teams use them where a flowchart would do but the process has parallel work, handoffs between roles or exceptions that need to be shown precisely.

When should you use an activity diagram?

• Describing a business process. Order fulfillment, onboarding and approval workflows all fit, especially with swimlanes for the roles involved.
• Detailing a use case. Each oval in a use case diagram can have an activity diagram showing how it runs.
• Showing parallel work. When two things happen at once and must both finish, an activity diagram says so with a fork and a join, which a flowchart cannot.
• Modeling before automating. Agree on the current process, including exceptions, before building a workflow engine or an integration around it.

What symbols does an activity diagram use?

Symbol	Shape	Meaning
Initial node	Filled circle	Where the activity starts
Action	Rounded rectangle	One step, named with a verb, such as “Charge payment”
Control flow	Arrow	The order actions run in
Decision	Diamond with one input and several outputs	One outgoing path is taken, chosen by guards in square brackets such as [yes]
Merge	Diamond with several inputs and one output	Brings alternative paths back together; no waiting
Fork	Thick bar with one input and several outputs	All outgoing paths start at once
Join	Thick bar with several inputs and one output	Waits for every incoming path before continuing
Activity final	Bullseye	The whole activity ends
Flow final	Circle with an X	One path ends, the rest continue
Object node	Rectangle	Data passed between actions, such as an order or an invoice
Swimlane	Column or row with a name	Who performs the actions inside it

The decision and merge nodes use the same diamond, and the fork and join use the same bar. You tell them apart by counting the arrows: one in and many out means decision or fork, many in and one out means merge or join.

How do you read an activity diagram?

Start at the filled circle and follow the arrows. In the order processing example, the first action is “Receive order”, which leads to a decision: is the item in stock? The guards on the outgoing arrows, [yes] and [no], say which path is taken.

On the [yes] path the flow hits a fork. Both arrows out of it start at once, so “Charge payment” and “Pick and pack” run in parallel. The join on the right waits until both have finished before “Ship order” begins. A flowchart would have to put payment before packing or after it, and either order would misstate how the warehouse works.

On the [no] path the flow goes to “Notify customer”. Both paths then arrive at a merge diamond, which passes whichever path arrives straight through. “Send confirmation” runs in both cases, and the bullseye ends the activity.

How do you create an activity diagram?

1. Set the start and end. Name the trigger and the outcome, and draw the initial and final nodes first.
2. List the actions in order. Start each with a verb. Stay at one level of detail.
3. Add decisions with guards. Every outgoing arrow from a decision gets a condition, and the conditions should not overlap.
4. Find the parallel work. Where two actions do not depend on each other, put them between a fork and a join.
5. Merge the branches. Where alternative paths come back together, use a merge node rather than pointing several arrows at one action.
6. Add swimlanes if more than one role is involved. Then every handoff is a line crossing a lane boundary.
7. Walk through a real case. Follow one order through and check it never gets stuck or takes two paths at once.

What are common activity diagram mistakes?

• A fork with no join. Parallel paths that never rejoin leave the reader unsure when the activity is done. Either join them or end each with a flow final.
• A decision used as a fork. If both branches must run, it is a fork. If one is chosen, it is a decision.
• Guards that overlap or leave a gap. [amount > 100] and [amount < 100] miss exactly 100. Use [else] for the remaining case.
• Missing exceptions. Out of stock, payment declined and address invalid are where the process gets interesting. Show at least the common ones.
• Treating it as a sequence diagram. If you need to show which component sends which message, draw a sequence diagram instead.

How does an activity diagram compare with other diagrams?

Diagram	What it shows	Use it when
Activity diagram	The flow of actions, with decisions and parallel paths	The process has parallelism, handoffs or needs UML precision
Flowchart	Steps and decisions in a loose, familiar notation	The audience is general and the process is simple
BPMN	Business processes in a richer, executable notation	The model has to drive a process engine or be audited
Sequence diagram	Messages between participants in time order	The question is who calls whom
State diagram	The states of one object and the events that change them	The question is about an object’s lifecycle, not a workflow

What tools can you use to make an activity diagram?

• Drag-and-drop diagramming tools. Most ship the UML activity shapes, including fork and join bars and swimlanes.
• Diagram-as-code tools. PlantUML has a full activity diagram syntax, and Mermaid flowcharts cover the simpler cases, so the diagram can live in the repository.
• AI diagramming tools. Tools like Eraser take a written process, a ticket or a procedure and return a UML diagram you can keep editing, which is the fastest way to get an existing process onto the page.

For more on this kind of diagram, see process flow diagrams.

Frequently asked questions